Have you ever unplugged the wrong drive and then had to rebuild the entire array?  It may not be a big deal in some ways but it does make your system vulnerable until the rebuild is done.

Many distros often enable the "bitmap" feature and this basically keeps track of what parts need to be resynced in the case of a temporary removal of a drive from the array, this way it only needs to sync what has changed.

To enable bitmap to speed up rebuilds and sync only the changed data

mdadm --grow /dev/md12 --bitmap=internal

Once you do that when you do a cat /proc/mdstat you\'ll see the change (with bitmap: being mentioned):


md12 : active raid1 sde1[0] sdd1[1]
      1953511936 blocks [2/2] [UU]
      bitmap: 233/233 pages [932KB], 4096KB chunk
 

Here's what happens when unpulgging a drive from  a RAID 1:

[46911.392690] raid1: sde1: rescheduling sector 1953247352
[46911.425512] raid1: Disk failure on sde1, disabling device.
[46911.425513] raid1: Operation continuing on 1 devices.
[46911.425529] raid1: sdd1: redirecting sector 1953247352 to another mirror
[46911.544209] RAID1 conf printout:
[46911.544213]  --- wd:1 rd:2
[46911.544215]  disk 0, wo:1, o:0, dev:sde1
[46911.544216]  disk 1, wo:0, o:1, dev:sdd1
[46911.560862] RAID1 conf printout:
[46911.560864]  --- wd:1 rd:2
[46911.560867]  disk 1, wo:0, o:1, dev:sdd1

When readding it, they were both still sync'd almost instantly, this is because no changes were really made.

md12 : active raid1 sdf1[0] sde1[2](F) sdd1[1]
      1953511936 blocks [2/2] [UU]
      bitmap: 0/233 pages [0KB], 4096KB chunk


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